A use of nitrogen which is becoming increasingly more important is as a fluid for use in secondary oil or gas recovery techniques. In such techniques a fluid is pumped into the ground to facilitate the removal of oil or gas from the ground. Nitrogen is often the fluid employed because it is relatively abundant and because it does not support combustion.
When nitrogen is employed in such enhanced oil or gas recovery techniques it is generally pumped into the ground at an elevated pressure which may be from 500 to 10,000 psia or more.
The production of nitrogen by the cryogenic separation of air is well known. One well known process employs two columns in heat exchange relation. One column is at a higher pressure in which the air is pre-separated into oxygen-enriched and nitrogen-rich fractions. The other column is at a lower pressure in which the final separation of the air into product is carried out. Such a double column process efficiently carries out the air separation and can recover a high percentage, up to about 90 percent, of the nitrogen in the feed. However such a process has a drawback when the nitrogen is desired for use in enhanced oil or gas recovery because the product nitrogen is at a relatively low pressure, generally between about 15-25 psia. This necessitates a significant amount of further compression of the nitrogen before it can be utilized in enhanced oil or gas recovery operations. This further compression is quite costly.
Also known are single column cryogenic air separation processes which produce high pressure nitrogen typically at a pressure of from about 70 to 90 psia. Nitrogen at such higher pressure significantly reduces the cost of pressurizing the nitrogen to the level necessary for enhanced oil and gas recovery operations over the cost of pressurizing the nitrogen product of a conventional double column separation. However, such single column processes can recover only a relatively low percentage, up to about 60 percent, of the nitrogen in the feed air. Furthermore, if one carried out the separation in the column at a higher pressure in order to produce nitrogen at a higher pressure than 70-90 psia, one would experience an even lower recovery than the 60 percent referred to above.
It is therefore an object of this invention to provide a cryogenic air separation process which will produce nitrogen at an elevated pressure and at a high separation efficiency and at high recovery.